In-phase and Quadrature phase signal imbalance, commonly referred to as IQ imbalance, occurs when the gains and/or phases differ in the In-phase and Quadrature paths of a zero-IF (a.k.a. direct down/up-conversion) transmitter or receiver. In the case of single-tone transmissions, the IQ imbalance manifests itself as an image tone at frequency fc−f when an intended signal is sent at frequency fc+f by the transmitter. DC offset occurs when the local oscillator, which is used to generate the carrier frequency either at the transmitter or at the receiver, leaks into the transmitted signal after upconversion or into the received signal before downconversion. At RF frequencies, the DC offset is manifested as a tone at the local oscillator frequency. If either of these impairments is left unmitigated, the achievable reliability of decoding/detection may, and often is, reduced.
IQ imbalance and DC offset typically occur both at the transmitter and at the receiver. A measurement procedure that can efficiently differentiate between transmitter-side impairments and receiver-side impairments would be useful since such measurements would allow for separate compensation to be performed depending on the source of the signal impairment. In many cellular applications, measurements and calibrations of IQ imbalance and/or DC offset are typically performed off-line before mobile device deployment. However, factory calibrations may be inaccurate in the field, e.g., due to component aging, temperature variation, power supply variation, environmental interference, and/or changes in the environment or operating conditions in which a device is used. In some cellular cases, measurements are done on-line wherein a mobile device listens to pilots transmitted by a base-station and bases its compensation upon those measurements. However, in such an approach only the combined effect of base-station transmitter's IQ imbalance and mobile device receiver's IQ imbalance is computed and obtained.
In wireless peer-to-peer networks, measuring and compensating both transmitter and receiver IQ imbalance corresponding to an individual device is very important. This is because there is typically a large dynamic range of signals operating concurrently in the same band. In addition in a peer to peer network, a device may communicate directly with a plurality of different devices in a relatively short period of time.
In view of the above, it should be appreciated that there is a need for methods and/or apparatus for measuring receiver and transmitter side induced IQ imbalance. It is desirable that at least some methods be suited for making such measurement after a device is deployed for use.